New research shows the two aren’t entirely separate.
Climate change’s oft ignored twin, ocean acidification, is usually thought of as a biological rather than a climatic problem. They’re seen as parallel (carbon dioxide emissions are a cause of each) but separate (the effects of ocean acidification don’t depend on changes in climate). Some recent studies are showing that, true to the interconnected nature of, well, nature, ocean acidification may actually have a climatic effect of its own.
Ocean acidification is a decrease in the pH and carbonate concentration of ocean water caused by CO2 pumped into the atmosphere. It’s generally bad news for critters with calcium carbonate shells or skeletons, and acidification has even been shown to affect fish. Studies in which CO2 is added to closely monitored sections of marine habitat have shown that one of the many outcomes appears to be a decrease in dimethylsulfide produced by phytoplankton.
This turns out to be pretty interesting, because this is the biggest source of biologically created sulfur that makes its way into the atmosphere, where sulfur compounds are hugely important for the formation of clouds. (They help create the cloud condensation nuclei that cloud droplets grow around.) Since cloud cover affects the amount of sunlight reflected back into space, this has the potential to affect climate.
But are we talking about a negligible impact or a significant one? A group of researchers set out to explore this question using a climate model developed by the Max Planck Institute for Meteorology in Hamburg, Germany. The models simulated the effect of an acidification-induced decline in sulfur production on clouds.
But the magnitude of that sulfur decline is highly uncertain. The handful of existing studies came up with different estimates of how much sulfur production drops as pH goes down. Rather than guess which estimate came closest to the truth, the researchers ran their simulations with high, medium, and low estimates.
One simulation left the link between acidification and sulfur out completely, providing a baseline for a comparison of warming by the end of the century with a middle-of-the-road emissions scenario. Then the model was run with the three estimates of acidification’s effect on sulfur.
In the baseline model, the flow of biologically created sulfur from the ocean to the atmosphere still decreased by seven percent because of climate change. Warming the surface ocean cuts down on mixing with deeper, nutrient-rich water, so phytoplankton productivity drops.
But in the simulations that included the impact of acidification, that sulfur contribution to the atmosphere decreased by 12 to 24 percent. The effect this has on cloud formation in the model is measured in terms of the additional energy from the Sun reaching the Earth’s surface—0.08 Watts per square meter due to warming the waters the phytoplankton live in and 0.18 to 0.64 Watts per square meter due to acidification. Allowing for uncertainty in exactly how sensitive the climate is to change, that equates to 0.1 to 0.76 °C of additional warming caused by ocean acidification at the end of the century. Keeping in mind that this emissions scenario projects around 2.8 °C of warming by 2100, that could potentially be a significant addition.
While climate change and ocean acidification are parallel phenomena, there are also some cross-links enabling the twins to interact. Rising temperatures and changing seawater chemistry will have impacts on marine life, and some of those impacts could, in turn, affect rising temperatures. That’s why it’s called the climate system—when you tug on one thing, many things move.
Nature Climate Change, 2013. DOI: 10.1038/NCLIMATE1981
Scott K. Johnson, ars technica, 29 August 2013. Article.
